This invention relates to a process for the extraction of aromatics from process gas streams containing various organic components such as tail gas streams from catalytic cracking, the hydrogen stream from catalytic reforming of naphthas, exit gas streams from petrochemical processing reactors, and the like. More particularly, this invention is concerned with the use of organic salts, in particular, quaternary organic ammonium, phosphonium, and arsonium salts with inorganic and organic anions, including halides and pseudohalides, as well as organic quaternary boranates, borates and aluminates as absorption agents, both alone and supported, for the organic or some of the organic components in the process gas streams, even at quite high temperatures. Because of the non-volatility of these materials as salts and their low solubility in hydrocarbon or polar liquids, these salts are also usable as selective sorbants for extraction of aromatics and other polar organics at lower temperatures when contacted with liquid process streams or waste water streams at lower temperatures. Such salts show the exceptionally low volatilities of salts, essentially nil, thus allowing their contact with large amounts of gases flowing at high rates without significant losses and facilitating the recovery of extractate or absorbate. The polarity of the materials as salts and their very large sizes and the essentially spherical shapes of the component ions make the materials essentially insoluble and such melting, as may take place, only seems to allow the materials to become tacky or semi-solid under operable absorption conditions. The materials have the exceptional thermal stability for the high temperature conditions and do not steam-distill as do more conventional organic absorbants.
In certain instances, it is desirable to remove organics to prevent pollution of the air and water. In other instances, it is desirable to selectively remove and recover aromatics or olefins from gas streams which are to be used as fuel for burning and which are too dilute for recovery by conventional means, and thus, the aromatics or olefins are wasted. In other instances, the hydrocarbon gases in the hydrogen off-gas from catalytic naphtha reforming are undesirable impurities and present absorption methods are less than entirely efficient. Furthermore, there are petrochemical gas streams where olefins are present in gas streams and it is desirable for selective recovery for recycle, but they are present in such dilute state that present methods are relatively rarely used and considerable quantities are lost to uses as fuel. Yet further liquid process feed streams are instances where the presence of trace aromatics or olefins can poison process catalysts, as in paraffin isomerization, and removal of the trace polar contaminants is most desirable and necessary, but present absorption and extraction methods are less than adequate and the usual feed preparation involves an expensive exhaustive hydrogenation step.